This invention relates to methods of producing metal containers or the like by pressure forming a hollow metal preform. In an important specific aspect, the invention is directed to methods of pressure-ram-forming aluminum or other metal containers having a contoured shape, such as a bottle shape with asymmetrical features.
Metal cans are well known and widely used for beverages. Conventional beverage can bodies generally have simple upright cylindrical side walls. It is sometimes desired, however, for reasons of aesthetics, consumer appeal and/or product identification, to impart a different and more complex shape to the side wall and/or bottom of a metal beverage container, and in particular, to provide a metal container with the shape of a bottle rather than an ordinary cylindrical can shape.
Methods have heretofore been proposed for producing such articles from hollow preforms by pressure forming, i.e., by placing the preform within a die and subjecting the preform to internal fluid pressure to expand the preform outwardly into contact with the die. As described, for example, in U.S. Pat. No. 6,802,196 and U.S. Pat. No. 7,107,804, the entire disclosures of which are incorporated herein by this reference, pressure-ram-forming (PRF) techniques provide convenient and effective methods of forming workpieces into bottle shapes or other complex shapes. Such procedures are capable of forming contoured container shapes that are not radially symmetrical, to enhance the variety of designs obtainable.
In a PRF method for forming a metal container of defined shape and lateral dimensions, a hollow metal preform having a closed end is disposed in a die cavity laterally enclosed by a die wall defining the shape and lateral dimensions, with a punch located at one end of the cavity and translatable into the cavity, the preform closed end being positioned in proximate facing relation to the punch and at least a portion of the preform being initially spaced inwardly from the die wall. The preform is subjected to internal fluid pressure to expand the preform outwardly into substantially full contact with the die wall, thereby to impart the defined shape and lateral dimensions to the preform, the fluid pressure exerting force, on the preform closed end, directed toward the aforesaid one end of the cavity. Either before or after the preform begins to expand but before expansion of the preform is complete, the punch is translated into the cavity to engage and displace the closed end of the preform in a direction opposite to the direction of force exerted by fluid pressure thereon, deforming the closed end of the preform. Translation of the punch is effected by a ram which is capable of applying sufficient force to the punch to displace and deform the preform. This method is referred to as pressure-ram-forming because the container is formed both by applied internal fluid pressure and by the translation of the punch by the ram.
The preform is a unitary workpiece typically having an open end opposite its closed end and a generally cylindrical wall. The punch has a contoured (e.g. domed) surface, and the closed end of the preform is deformed so as to conform thereto. The defined shape, in which the container is formed, may be a bottle shape including a neck portion and a body portion larger in lateral dimensions than the neck portion, the die cavity having a long axis, the preform having a long axis and being disposed substantially coaxially within the cavity, and the punch being translatable along the long axis of the cavity.
Also, advantageously and preferably, the die wall comprises a split die separable for removal of the formed container, i.e., a die made up of two or more mating segments around the periphery of the die cavity. With a split die, the defined shape may be asymmetric about the long axis of the cavity.
The PRF operation is desirably performed with the preform at an elevated temperature. In addition, it has heretofore been proposed to induce a temperature gradient in the preform, for example by adding separate heaters for inducing a temperature gradient in the preform from the open end to the closed end. Such a temperature gradient in the preform helps control the onset of preform expansion (bulging) when internal fluid pressure is applied to the preform within the die. Specifically, an open-to-closed end pressure gradient causes progressive expansion wherein the portion of the preform adjacent the open end, being at a relatively higher temperature, bulges out first until it comes into contact with the die, thus locking the preform in the die cavity as expansion moves toward the closed end, while the backing ram pushes the punch toward and holds contact with the closed end of the preform to form the closed end (container base) profile. In particular, progressive expansion prevents blow-outs by allowing the ram to move the punch into contact with the closed end and form the container base before the adjacent part of the preform engages the die wall.
It is difficult to control a temperature gradient in the preform, however, because the gradient can be adversely affected by variables such as production speed, preform size and tooling set-up. Thus, it would be advantageous to achieve the benefits of progressive expansion from open end to closed end without the necessity of establishing and maintaining a temperature gradient effective for that purpose.